1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, and more particularly to a system which performs a duty ratio control of an open/close valve disposed in an air intake side secondary air supply passage.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are well known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and the air-fuel ratio of the mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and an improvement of the fuel economy. As an example of the air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533. In this air intake side secondary air supply system, an open/close valve is disposed in an air intake side secondary air supply passage which communicates with the carburetor on the downstream side of the throttle valve, and the open/close valve is on-off controlled in response to the output signal level of the O.sub.2 sensor, so as to effect a "duty ratio control" of the supply of the air intake side secondary air. In conventional air intake side secondary air supply systems as the above, it is general to set an open/close duty ratio of the open/close valve only in response to a result of a comparison between the output signal level of the O.sub.2 sensor and a level corresponding to a target air-fuel ratio. For this reason, a delay of the response of the feedback control tends to become large especially when the engine operation enters into a low load condition. This delay of response corresponds to the time required for the detection of the supply of the air intake side secondary air by means of the O.sub.2 sensor in the form of a change in the oxygen concentration in the exhaust gas. As a result, it is difficult to avoid a hunting of the air-fuel ratio from the target air-fuel ratio, which in turn has been causing a deterioration of the driveability of the engine and an increase of noxious components in the exhaust gas.